Thompson center encore 300 win mag

[88mustang]

I have tc encore in 300 win mag with leuplod base and rings. The problem the gun starts out shooting good and then will put a bullet out of the group. I remove all of the fouling in the barrel and have the same problem
I have tried Remington cor-lok 150 180
Hornaday sst 150
Hornaday American whitetail 180
Hornaday precision 200
The hornaday precision shots the best
With a total of probably 120 rounds